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Pentaquark: Fact or FancyPatrizia Rossi
Laboratori Nazionali di Frascati - INFN
•Introduction•Experimental Overview•Comments on positive and null results•Dedicated Experimental Program @ JLab•Conclusion
Outline
The CLAS collaboration has decided that results will be presented
only when finalNormally, preliminary results would be shown,
but these are not normal circumstances.
•Baryons whose minimum quark content is 4 quarks and 1 antiquark•“Exotic” pentaquarks are those where the antiquark has a different flavor than the other 4 quarks ; quantum num. cannot be defined by only 3 quarks
Pentaquarks
Introduction
QCD: predicts the existence of mesons and baryons with more complicated internal structure.
non-conventional mesons : ggg(glueballs), qqg (hybrids),qqqq non-conventionale baryons: qqqg, qqqqq-
- --
Known HADRONSbound states of three quarks(qqq) : BARYONSquark and anti-quark(q q) : MESONS
Baryon number = 1/3 + 1/3 + 1/3 + 1/3 – 1/3 = 1Strangeness = 0 + 0 + 0 1 + 1 = 0
Ex: uudss, non-exotic
Baryon number = 1/3 + 1/3 + 1/3 + 1/3 – 1/3 = 1Strangeness = 0 + 0 + 0 + 0 + 1 = +1
Ex: uudds, exotic
Introduction
Before 2003
There were searches in the 1960’s and1970’s mostly in bubblechamber experiments with incident π and K beams for what wasthen referred to as the Z , a baryon with positive strangeness.Those experiments found no enhancements in S=+1 baryon channels.Indeed the failure to find such flavor exotic baryons lent credenceto the then nascent quark model.
After 2003
…..Hadronic Physics has beenvery interesting
S = +1
S = 0
S = -1
S = -2
G < 15 MeV
Spectacular Development
1987 First prediction of the mass: Mass Z+ (uudds*) = 1530 MeVPreszalowicz, Chiral Soliton Model, World Scient. ’87, p. 112
1997 First prediction of the width: Γ < 15 MeVDiakonov, Petrov, Polyakov, Chiral Soliton Model, Z. Phys. A 359(1997)305 Mass of Z+ (uudds*) = 1530 MeV, Spin=1/2, Isospin=0, Parity +, S=+1.
The width prediction was the key for theexperimental discovery of a Θ+ candidate in 2003
2003 First observation of Z+ renamed Θ+: Mass 1540 ±10 MeVLEPS Collaboration, T Nakano et al, PRL91 (2003) 012002
Photoproduction on boundneutron in 12C g n → Q+ K- Q+ → K+ nQ+ signal in the invariantmass of K+n system
NQ = 19.0±2.8NB = 17.0±2.8MQ = 1.54 ± 0.01 GeVGQ(FWHM) = 25 MeVGaussian sig. S= 4.6s
Q+ peak
Pentaquark in the Chiral Quark Soliton Model
Definite predictions about masses & widths of the states
Presence of three states with explicit exotic quantum numbers
Width very narrow → directly visible in measured invariant masses → No complicate p.w.a.needed
-• Soliton: rigid core (q3) surrounded by meson fiels (qq)• Barions are rotational states of the soliton nucleon in spin-isospin space
-1st eccited state : [8] J=1/22nd “ : [10] J=3/23rd “ : [10] J=1/2 exotic ( N*(1710) fixes all the masses)
S = +1
S = 0
S = -1
S = -2
G < 15 MeV
G ~ 140 MeV
•Diakonov et al. - Z. Phys. A359 (1997) 305•D. Diakonov, V. Petrov, arXiv:hep-ph/0310212
Symmetries give an equal spacingbetween “tiers”
Where we are now
Before 2003 searches for flavor exotic baryonscame up empty but now we have claims for:
X5 X50
Evidence for Pentaquark StatesSpring8 SAPHIR
JLab-p
HERMES
ITEP
pp S +Q +.
COSY-TOFDIANA
SVD/IHEP
JLab-d
ZEUSCERN/NA49
H1
This is a lot of evidence
Positive Results
Ν5GRAALTalk@penta2004 Conf
Ν5 Ξ05STAR
hep-ex/0406032
pp → K0pΣ+ Θ+
K0 pCOSYPhys. Lett. B595(2004)127
ep → PX ΘcH1Phys. Lett. B588(2004)17
ep → PX Θ+
K0 pZEUSPhys. Lett. B591(2004)7
pp → Ξ π X Ξ5
NA49PRL 92(2004) 042003
γ∗d → K0pX Θ+
K0 pHERMESPhys. Lett. B585(2004)213
νC → K0pX Θ+
K0 pNOMADTalk@neutrino 2004Conference
K+ Xe → K0 pXe’ Θ+
K0 pDIANAPhys. Atom. Nucl. 66(2003) 1715
p+C3H8 → K0pX Θ+
np →npK+K- K0 pJINRhep-ex/0403044hep-ex/0404003hep-ex/0410016
γp → K0 K+n Θ+
K+ nSAPHIRPhys. Lett. B572(2003)127
νA → K0pX Θ+
K0 pITEP (ν)Phys. Atom. Nucl. 67(2004) 682
γd → K+ K-np Θ+
γp → K+ K-nπ+ K+ nCLASPRL 91(2003) 252001PRL 92(2004) 032001
pA → K0pX Θ+
K0 pSVDhep-ex/0401024
γC12 → K- K+n Θ+
K+ nLEPSPRL 91(2003) 012002
Q+ : Mass & Width
Mass (MeV) FWHM (MeV)
~12 MeV
Mass
•Systematic shift in the ϑ+ mass for the 2 decay modes: nK+ and pK0
•Ranges from ≈1520 to ≈1555 MeV/cWorld average: 1532.2 ± 1.3 MeV/c
Width
•Most measurements are only upperlimits
•FWHM ranges from ≈10 to ≈25 MeV/cdominated by experimental resolution
•Arndt et al. and Cahn et al. analysis of KN phase shifts suggests thatΓ < 1 MeV !!
BaBar
BELLE
ΛDELPHISPHINX
E690
SELEX
L3
ALEPH
HyperCP
Null Θ+ States
FOCUS
BES
HERA-B
PHENIX
CDF
LASS
Negative Results
AuAu → PX Θ+PHENIXnuc-ex/0404001
e+e- → ϒ(4S) Θ+
Ξ5
BaBarhep-ex/0408064
pC(N) → ΘKC(N) Θ+
SPHINXEur. Phys. J. A21(2004) 455
KN → PX Θ+
Θc
BELLEhep-ex/0411005
pA → PX Θ+
Ξ5
HERA-BPRL 93(2003) 212003
e+e- → J/ψ(ψ(2S)) Θ+BESPRD 70 (2004) 012004
ep → PX
Θc Ξ5
ZEUShep-ex/0410029/0412005hep-ex/0412014
pp → PX Θ+
Ξ5
E690Quark Confinement 2004
Σ-N→PX Ξ5
WA89PRC 70 (2004) 022201
γp → PX Θ+
Θc Ξ5
FOCUShep-ex/0412021
γ γ → ΘΘbar Θ+L3hep-ex/0410080
(π,p, Σ)p → PX Θ+SELEXQuark Confinement 2004
Hadronic Z decays Θ+DELPHIhep-ex/0410080
(π+,K+,p)Cu→PX Θ+HyperCPPRD 70 (2004) 111101
Hadronic Z decays Θ+
Θc Ξ5
ALEPHPhys. Lett. B599(2004)1
ppbar → PX Θ+
Θc Ξ5
CDFhep-ex/0408025hep-ex/0410024
Comments on positive and null results
There are valid criticisms for both positive and null experimental results
Critical View on Positive Experiments
For most of the experiments, the background shapeis not clearly knownSome experiments have harsh cuts that could affectthe spectra unexpectedlyor specific reaction mechanism are assumedcuts are necessary to lower the backgroundKinematic Reflections?Almost all experiments have less than 100 signalevents (except ZEUS)
Background
Background due to the non-resonant K+K- production in the region above 1.59 GeV fitted by a distribution ofevents from LH2 target and scaled by a factor 0.2
Q+
LEPS
Monte Carlo calculation•three (pK+K-) and four-body (pK+K-n) phase-space photoproduction•full simulation of detector response•BKG spectrum fitted to the data
Using Monte Carlo calculation: σ=4.8Using different fitting functions: σ=4.6÷5.8
simulated BKG
fitted BKG
CLAS(d)
Background
FRITIOF (hadron-hadron, hadron-nucleusinteractions generator) fails to reproducethe real background shape. This may be causedby the bunch of Σ*+ bumps which have ratherhigh branching ratio for pK0 decays.
Distribution fitted by Gaussian function anda fourth-order polynomial background.
SVD
Background
0
Monte Carlo calculation:• non-resonant contribution
from PYTHIA code• six S+* broad resonances
decaying into K0p• one narrow resonance
near 1.53 GeV• Quite good description of
the spectrum
HERMES
Further cuts are motivated by assumptions on production mechanism
CLAS - Exclusive Production on Hydrogen (I)
• Data taken in 1999 and 2000 runs• Tagged photons 3.0 < Eg < 5.25gp → p+ K- Q+
→ n K+
After PID No cuts
M(nK+)
PRL 92, 032001-1 (2004)
M(nK+)
NQ = 41MQ = 1555 ± 1 ± 10 MeVGQ(FWHM) = 26 MeVS= (7.8 ± 1.0)s
CLAS - Exclusive Production on Hydrogen (II)
Possible production mechanism •Select t-channel process by tagging forward p+ and reducingK- from t-channel processes
cos ϑ* p+ >0.8cos ϑ* K+ <0.6
(in c.m. frame)
CLAS - Exclusive Production from Deuterium
gd → p K- Q+ → n K+
First exclusive measurement•Data taken in 1999 run•Tagged photons with up to 3 GeV•Charged particle detected in CLAS•Neutron ID by missing mass
Possible reaction mechanism
Requires FSIboth nucleons involved
DIANA
Inv mass(K0sp) without cuts to
reduce rescattering
!
s
b= 2.6"
Inv mass(K0s p) with cuts to reduce
rescattering − θp, θK<100o
− cos ΦpK < 0 (K0s and p back to back in
the plane transverse to the beam direction)
!
s
b=29
44= 4.4"
Kinematic Reflections
High mass mesonsa2(1320), f2(1270)decaying to K+K-
can producestructures in M(NK)at low mass (A. Dzierba et al., hep-ph/0311125)
Dzierba’s calculationCLAS data on d
K. Hicks et al., hep-ph/0411265found inconsistencies inDzierba’s approach
Kinematic Reflections
mKK = m(a2) & |Y2±1(ϑ,φ)|2
mKK = m(ρ3) & |Y3±1(ϑ,φ)|2
Signals Statistical Significance
Experiments Signal Background Published Significance s b ξ1 ξ2
LEPS 19 17 4.6 4.6 3.2 DIANA 29 44 4.4 4.4 3.4 CLAS(d) 43 54 5.2 5.9 4.4CLAS(p) 41 28 7.8 7.8 4.9 SAPHIR 55 56 4.8 7.3 5.2COSY 60 103 4-6 5.9 4.7 SVD 50 78 5.5 5.6 4.4νBC 19 8 6.7 6.7 3.7HERMES 52 155 4.3-6.2 4.2 3.6ZEUS 230 1080 4.6 7.0 6.4 NOMAD 33 59 4.3 4.3 3.4
1. Q+ peak is evident only for Q2 > 20 GeV2.ZEUS suggests that this condition gives the Q+ enoughtransverse momentum to get into their detector acceptance.
2. BackgroundMC simulation using ARIADNEgen. does not describe well data <1.6 GeVThe PDG reports a Σ bump at 1480 MeVand several states above 1550 MeV.None of these are included in the simulation which includes only wellestablished resonancesBackground fit with a 3-par function + 2 gaussian
Results from ZEUS
Phys. Lett. B591(2004)7
Critical View on Negative Experiments
All negative results are from high energy experiments: inclusive versus exclusive measurement
-inclusive has better resolution but more background
Production mechanisms are not generally known-sensitivity to the pentaquarks may be highly suppressed in thecurrent fragmentation
Negative Results for Θ+ in e+ e-
Exp (GeV) Reaction Statistics Upper Limit
!
s
ψ(2S) - J/ψ → single Θ pro. < ~ 10-5
BES 3.7 e+e-→ψ(2S)→ΘΘ 14x106
J/ψ 58x106
BaBar 10.5 e+e-→ϒ(4S)→BB 135x106
ALEPH ~91 e+e-→Z0→qq→ΘX 4x106-
-
-ψ(2S)→ΘΘ→K0pK-n + K0pK+n < 0.84 x 10-5- - -J/ψ→ΘΘ→K0pK-n + K0pK+n < 1.1 x 10-5- - -
e+e-→qq→Θ X < 5-10 x 10-5-
e+e-→Z0→Θ X < 0.6 x 10-5
BESPRD 70 (2004) 012004
Hadron Rate in e+e-
e+e- experiments may be not sensitive to pentaquark signal
Slope forPentaquark?
BaBar limit
Slope forMesons
Slope forBaryons
Hadron Rate in e+e-
Q5+
qqqqqe- e+
Current fragmentation
Θ+ = baryon containing a strange antiquark
a) Low energy photoproduction experiment :baryon (target) - strange antiquark (strange component of the γ)b) e+e- experimentsB number and S must be created from gluons
How to normalize Θ+ production in b) with respect to a) ?
Measuring B-B and Υ-Υ in the same exp that does not see the Θ+
Tuning: measure the ratio d/p in a given experiment
- ---
d/p (H1) = 5.0±1.0±0.5 x 10-4--
d/p (OPAL) = 0.8 x 10-5 (90% C.L.)--
Inclusive Production (fragmentation region)
fragmentation
recombination
soft interactionA
B
h
i
9 x 10-4 [quark]3 x 10-2 [diquark]RΘY ~ {0.11(1-z)4 [quarks]
0.30(1-z)2 [diquarks]RΘY ~ {
A B→Θ+XA B→Y X
Titov et al. nucl-th/0408001X-section estimation on the base offragmentation-recombination model
<0.03 L(1520)pp* --> pKs0XCDF
<0.02 L(1520)pA --> pKs0XHERA-B
!
y =ph
pi
!
Rhi
(y);
Scale behavior of F & R
!
FiA
(x) = F0(x)(1" x)
bA
i
!
Rhi
(y) = R0(y)(1" y)
cih
F0 and R0 smooth function of x and y ; keeping the dominant term the an beperformed by an elementary method; z = 0.7 typical value for the fragm region
!
" # FiA
$ (x)Rhi
(y)%(z & xy)dxdy
!
z =ph
pA;
!
FiA
(x)
!
x =pi
pA;
Theoretical Development
Conclusion on the experimental review
PDG july 2004….. Θ(1540)+
Guidance from the wisdom of Feynman and Wigner1. Feynman told us that we learned from sharpening contradictions
2. Wigner told us that few free parameters can fit an elephant. Afew more can make him wiggle his trunk
3. Wigner’s response to questions about a particular theory he did notlike was:” I think that this theory is wrong. But the old Bohr -Sommerfield quantum theory was also wrong. Could we havereached the right theory without going through that stage?”
H. Lipkin hep-ph/0501209
Goal of the Experimental 5-quark Program @ JLab
We need conclusive confirmation of the Q+
Experiments at low energies, with much higher statisticswill settle the question
Goal of the Experimental 5-quark Program @ JLab
•High statistics search for Q+
•Solving the issues of Q+ (1540)
•Search for excited states of the Q+ (1540)
•Search for the other exotic members of the 10plet X5--,X5
+?
Beam
Superconducting recirculatingelectron accelerator
JLab Accelerator CEBAF
Hall B
• Continuous Electron Beam• Energy 0.8-5.7 GeV• 200mA, polarization 75%• Simultaneous delivery to 3Halls
Hall B: Cebaf Large Acceptance Spectrometer + Tagger
Torus magnet6 superconducting coils Electromagnetic calorimeters
Lead/scintillator, 1296 photomultipliers
beam
Drift chambersargon/CO2 gas, 35,000 cells
Time-of-flight countersplastic scintillators, 684 photomultipliers
Gas Cherenkov counterse/p separation, 256 PMTs
Liquid D2 (H2)target +g start counter; e minitorus
• Broad angular coverage (8° ÷ 140° in LAB frame)• Charged particle momentum resolution ~0.5% forward dir
• Tagged photon beam with energy resolution dk/k ~ 0.1%
CLAS is designedto measure exclusive reactions with multi-particle final states
•Eg = (20% - 95%) Ee
Current Projects @ Jlab
Eg : 1.5-5.7
Ee : 5.7
Eg : 1.6-3.8
Eg : 0.8-3.6
Ee : 1.5-5.4
Not yetscheduled
gpΞ5
Q+JLab/Hall-B(E-04-017)SUPER-G
Dec 05 2004 -January 31 2005
γdΞ5JLab/Hall-B(E-04-010)EG3
May 22 - July 262004
gpQ+
Q+*
Q++
JLab/Hall-B(E-04-021)G11
March 13-May 162004
gdQ+JLab/Hall-B(E-03-113)G10
May 22 - June 12004
epep
Q++
S05
JLab/Hall A(E-04-012)
•Data taking March 13 - May 16, 2004•Eγ = 0.8 - 3.59 GeV•B field = 2 settings of the torus magnetI = 2250A : Total of 316 runs, 6015 files Increased acceptance for forward going negative particles for inclusive (K+K-) and (KSp) analysisI = 3375A : Total of 300 runs, 6496 files Similar geometrical acceptance and single track resolution as for the published data•Target = 24 cm liquid deuterium
•10 billion of events collected ~ 50 pb-1 (10x statistic publ.results)
G10: Search for Θ+ on Deuterium
Data Taking
G10: Search for Θ+ on Deuterium
Data Processing Status
End of January 2005: final data processing → bos files
and ntuples with 4-vectors ready for physics analysis
Physics analysis tools:oAnalysis programs – mostly ready;
oDevelopment of GEANT based simulation tools are in progress.
Reactionsto study
γd→nK+K-p Θ+→nK+
γd pK0 K-(p) Θ+→pK0 K0 π+π-γd ΛK+n Θ+→nK+ Λ→ pπ-
γd ΛK0p Θ+→pK0 Λ→ pπ- K0→π+π-
•Independent analysis of several reactions by different groups
γ d → Θ+ Λ
(n)
(K+)
d
Decay modes: Θ+ K0p Λ pπ- K0 π+π-
Θ+ K+n Λ pπ-
• No possibility of kinematical reflections (A.R. Dzierba et al., hep-ph/0311125)
•S=+1 both for nK+ and pK0, thanks to Λ
G2a and G10 Comparison
G2 Data SetEe = 2.478 GeV (2/3 of G2a)Eγ < 2.35 GeVTorus Current = 3375 ATarget 10 cm LD2; center of CLAS
G10 Data SetEe = 3.775 GeVEγ < 2.35 GeVTorus Current = 3375 ATarget 24 cm LD2; -25 cm upstream ofCLAS center
γd → pK+K-(n)
Missing NeutronG2 G10
G2a and G10 Comparison
Λ(1520) and φ
•Data taking May 22 - July 26, 2004•Eγ = 1.6 - 3.8 GeV•Target = 40 cm liquid hydrogen•Events collected ~ 80 pb-1 (10x statistic published results)
G11: Search for Θ+ on Proton
Data Taking
Data Processing Status
End of February 2005: final data processing → bos files and
ntuples with 4-vectors ready for physics analysis
Reactionsto study
g+p Q+ + K0 K++ n (K0+ p) p++ p-
g+p X+++ K- K++ p + K-
g+p Q+ + K- + p+ K++ n (K0+ p) + K- + p+
G11: Search for Θ+ on Proton
γp → K+ (n) π+ π-
K0
n
M = 497.9 MeVσ = 4.3 MeV
M = 938.0 MeVσ = 9.4 MeV
G11: Search for Θ+ on Proton
γp → p ω
Development and test of the analysis procedure through thecross section extraction for known reactions
→ p p + (p - p 0 ) Topology 1→ p p + p - (p 0 ) Topology 2
Differential cross section
G11: Search for Θ+ on Proton
γp → K+ Λ(1116)
→ K+ p (p - ) Topology 1 L as MMK+→ K+ p p - Topology 2 L as IMpp-
Differential cross section
G11: Search for Θ+ on Proton
γp → K+ Λ(1116)
γp → p ωTotal cross sections
• NA49 experiment suggestspossibility of pentaquarkcascades.– Such states are predicted
as members of baryonantidecuplet.
– Small statisticalsignificance.
– The only evidence forsuch a state.
EG3: Search for Pentaquark Cascades
EG3: Search for Pentaquark Cascades
• CLAS allows simultaneous detection of multiple particles inthe final state. In particular channel
can be studied with CLAS.
Photon beam
Two decay vertices,Negatives bend outwards
X ct = 4.9 cmL ct = 7.9 cm<gb> ~ 1.5
EG3: Search for Pentaquark Cascades
Data Taking
•Data taking Dec 5 2005- January 31, 2005•Eγ = 4.4 - 5.5 GeV•Target = 40 cm liquid deuterium•Events collected ~ 4.2 Billion
Analysis Plan
Develop procedures to identify detached vertices with ~1cmresolution.
Identify Λ using (pπ-) missing mass.Identify Ξ- using (Λπ-) invariant mass, DOCA and flight path.Plot invariant mass of (Ξ- π-) to find Ξ-- peak.Develop/utilize kinematical fitting procedure to enhance peaks and
use confidence level cuts.
New data: LEPS deuterium
MMgK+ (GeV) MMgK- (GeV)
PreliminaryPreliminary
Q+
L(1520)
Minimal cuts: vertex, MMgKK=MN, no f , Eg < 2.35 GeV
Hope with CLAS to give soon a final answer on the pentaquark issue
